Sodium

The hair sodium test (sodium in hair; sodium, hair) is used to appraise longer-term nutritional and mineral status by leveraging hair as a time-integrating matrix. It serves as an adjunct in the evaluation of chronic alterations in sodium balance and may provide supportive information when trichothiodystrophy is suspected. This assay is not intended for assessment of acute electrolyte disorders.

Sodium is a macroelement and the predominant cation of extracellular fluid. Along with potassium, chloride, and bicarbonate, it maintains water and electrolyte balance, supports acid–base equilibrium, and enables normal neuromuscular function. Dietary intake is mainly from sodium chloride and sodium bicarbonate, and whole‑body sodium homeostasis is regulated chiefly by renal excretion under renin–angiotensin–aldosterone control. Hair incorporates sodium and other elements over approximately 6–8 weeks of growth, allowing evaluation of longer-term mineral balance relative to point-in-time blood or urine measurements. In dermatologic and metabolic contexts, hair sodium is an adjunctive marker and can be included in the workup of trichothiodystrophy. Interpretation is constrained by interindividual differences in hair structure and composition and by substantial effects of thermal or chemical hair treatments on measured sodium. Hair sodium measurement does not diagnose acute disorders of sodium metabolism, and results should be integrated with clinical assessment and other laboratory data.